Method for formation of a titanium carbide surface layer on titanium and titanium alloy parts

ABSTRACT

A METHOD OF FORMATION OF A LAYER OF TITANIUM CARBIDE AT THE SURFACE OF A COMPONENT OF TITANIUM OR TITANIUM ALLOY IN WHICH THE SAID COMPONENT IS ASSOCIATED WITH A FLUID MEDIUM HAVING A BASE OF A CARBON COMPOUND SUCH AS CARBON DISULPHIDE OR CARBON TETRACHLORIDE OR A MIXTURE OF THESE TWO, AND IS SUBJECTED IN SAID MEDIUM TO A HEAT TREATMENT OF SHORT DURATION BY HIGH FREQNENCY CURRENT, BRINGING THE SURFACE OF SAID COMPONENT TO A TEMPERATURE HIGHER THAN 1100*C. AND PREFERABLY HIGHER THAN 1350* C. THE INVENTION ALSO COMPRISES THE COMPONENTS OR PARTS MADE BY THIS METHOD.

Jan. 5, 1971 M. M. M. ADER 3,553,038

METHOD FOR FORMATION OF A TITANIUM CARBIDE SURFACE LAYER ON TITANIUM ANDTITANIUM ALLOY PARTS Filed 001,. 21 1966 v 4 Sheets-Sheet l 13% 14 o: 10o o o oo o o MA: r/lv M02054 Mme/E 4.05,

62% v JW/vv Jan. 5, 1971 M M ADER 3,553,938

METHOD FOR FORMATION OF A T ITAN.[UM CARBIDE SURFACE LAYER ON TITANIUMAND TITANIUM"ALLOY PARTS Filed Oct. 21, 1968 4 sheets-Sheet 2 FIG.2 H923O0 OQQZS oo 00 00 0 6 oo oo 00 0o 00 Iv E7 00 00 00 0o 00 0o oo oo M. MM. ADER Jan. 5, 1971 METHOD FOR FORMATION OF A TITANIUM CARBIDE SURFACELAYFH ON TITANIUM AND TITANIUM ALLOY PARTS 4 Sheets-Sheet 5 Filed Oct. I2l i968 FlG.3b

FIGS

FIG.3d

M42 m/ Mama MAW/5 405/6 Jan. 5, 1971 Filed Oct. 21

TITANIUM AND TITANIUM ALLOY PARTS 1966 4 Sheets-Sheet 4 M M. M. ADERMETHOD FOR FORMATION OF .A TITANIUM CARBIDE SURFACE LAYER ON FIG/la.

Awm rm M/MTl/V M40051 Mme/5 405g United States Patent 3,553,038 METHODFOR FORMATION OF A TITANIUM CAR- BIDE SURFACE LAYER 0N TITANIUM ANDTITANIUM ALLOY PARTS Martin Marcel Marie Ader, Paris, France, assignorto Centre Stephanois de Recherches Mecaniqnes Hydromecanique etFrottement, Saint-Etienne, France Filed Oct. 21, 1968, Ser. No. 769,302Int. Cl. C23c 9/10, 11/10 US. Cl. 148-20.3 19 Claims ABSTRACT OF THEDISCLOSURE A method of formation of a layer of titanium carbide at thesurface of a component of titanium or titanium alloy in which the saidcomponent is associated with a fluid medium having a base of a carboncompound such as carbon disulphide or carbon tetrachloride or a mixtureof these two, and is subjected in said medium to a heat treatment ofshort duration by high frequency current, bringing the surface of saidcomponent to a temperature higher than 1100 C. and preferably higherthan 1350" C. The invention also comprises the components or parts madeby this method.

The present invention relates to a method of commercial production of alayer of titanium carbide on the surface of a part of titanium or oftitanium alloy.

At the present time, the use of titanium in mechanical engineering isbecoming increasingly developed, but its applications are limited by thetwo following particular characteristics of titanium:

(1) Its easy and very rapid oxidation at temperatures below 500 C.;

(2) Its poor frictional qualities.

These two defects can be remedied, that is to say the oxidation of atitanium surface can be reduced and its seizure under considerablemechanical forces can be prevented, by proceeding to conversion of thesuperficial layers of the metal to titanium carbide.

Up to the present time, various methods have been proposed which allhave the disadvantages, on the one hand of being too complicated and tooexpensive to have profitable industrial applications, and on the otherhand of only permitting the production of layers of a thickness of theorder of one micron or even a few fractions of a micron, which areentirely inadequate for parts in the usual applications.

The known methods generally comprise prolonged heating for at least fourhours at a temperature below 1100 C.

The present invention has for its object a simple method ofcarburization of a part made of titanium or titanium alloy, to asuitable thickness.

According to the invention, a method of obtaining a layer of titaniumcarbide on the surface of a part of titanium or titanium alloy ischaracterized in that the part of titanium or titanium alloy issubjected, in a fluid medium with a base of a carbon compound such ascarbon sulphide or carbon tetrachloride or a mixture of both, to a heattreatment of short duration by high frequency, bringing its surface to atemperature higher than 1100 C. and preferably higher than 1350 C.

The medium with a base of carbon sulphide and/or carbon tetrachloride isfree from the drawbacks of hydrocarbons, in which the hydrogen is apoison for titanium, of oxygenated compounds which would form harmfuloxides at a temperature above 1100 C., and of certain dangerous andcostly halogenides.

The medium with a base of carbon sulphide and/or tetrachloride may beliquid or gaseous. The carbon sul- 3,553,038 Patented Jan. 5, 1971 phideand carbon tetrachloride are in the liquid state at ordinarytemperatures and are gaseous at temperatures exceeding 50 C. They maytherefore be utilized in the liquid or gaseous state, diluted or not ina neutral gas. It may be advantageous to add to the medium a suspensionof graphite, of sulphur or of alkaline carbides. Furthermore, adiscontinuous supply of the carburizing agent may be favourable.

The method according to the invention may be carried out in various waysand in particular in the following forms of execution:

(l) The carburizing agent comprises carbon sulphide and/or carbontetrachloride in the liquid state, filling a chamber which receives thepart to be treated;

(2) The carburizing agent comprises carbon sulphide and/or carbontetrachloride in the liquid state filling the chamber with agitationwhile receiving in suspension, carbon, sulphur or alkaline carbides;

(3) The carburizing agent (carbon sulphide or carbon tetrachloride or amixture of the two) in the liquid state is spread in the form of ashower into the interior of the chamber;

(4) The carburizing agent passes through the chamber in the vapour stateat a temperature higher than 50 C.;

(5) The carburizing agent in the vapour state is conveyed through thechamber by a flow of neutral gas at a temperature higher than 50 C.;

(6) The carburizing agent in the vapour state, at a temperature higherthan 50 (1., passes through a bed of graphite, sulphur or alkalinecarbides, thus creating in the chamber a fluidized bed of theseelements;

(7) The carburizing agent in the vapour state, conveyed by a flow ofneutral gas at a temperature higher than 50 C., passes through a bed ofgraphite, sulphur or alkaline carbides, thus producing a fluidized bedof these elements;

(8) The carburizing agent is sent by short and repeated puffs into aflow of neutral gas at a temperature higher than 50 C., which passesthrough the chamber;

(9) The carburizing agent in the vapour state, diluted in a neutral gasat a temperature higher than 50 C., is sent into the chamber by shortand repeated puffs.

According to a further characteristic feature of the invention, theheating of the part is obtained by electric currents induced in the bodyof the part by an appropriate inductor through which passes an electriccurrent having a frequency of at least 0.2 megacycle and preferably ofat least 0.4 megacycle, supplying a power of at least 40 kw./sq. dm. fora pre-determined short duration not exceeding 3 to 5 minutes andpreferably seconds.

In one form of embodiment the heating is carried out in a singlecontinuous operation, the power being applied to the inductorcontinuously during the short predetermined period.

In an alternative form, the heating is effected in a discontinuousmanner by applying the power to the inductor for short periods atregular intervals, the total time of application of the power to theinductor not exceeding the pre-determined short period.

When the method according to the invention is used to treat the part oftitanium or titanium alloy, it is found that with a very simpleapparatus, with products of low cost and a very brief heating time athigh frequency, there are obtained layers of titanium carbide which areperfectly adherent, regular, homogeneous, having a hardness greater than1.800 HV and having a thickness exceeding 5 microns. The parts thustreated have remarkable properties of resistance both to seizure and tooxidation.

In addition, the heating is so short that the heat does not have thetime to penetrate into the interior of the 3 part. The structure of thetitanium thus remains intact beyond a small depth, and the mechanicalproperties of elongation, striction, resilience and elastic limit areonly reduced to a negligible extent.

A test sample of titanium alloy having 6% of aluminium and 4% ofvanadium, 6 mm. in diameter and 50 cm. in length, treated by the methodaccording to the invention and set in rotation at 760 r.p.m., grippedbetween two jaws of 35 NCD 6 steel with a load of 1000 Newtons,

rotates for 250 seconds before creeping, without either cooling orlubrication, whereas the same sample untreated seizes after 1 to secondsof rotation under the same conditions.

In accordance with other arrangements of the invention, the treatmentproper is preceded by a pre-treatment and/ or a surface preparation.

In one method of carrying out the pro-treatment, the part is heated to atemperature comprised between 600 and 850 C. for a time comprisedbetween 1 and 20 hours in a chamber filled with graphite, which producesa saturation in carbon in the dissolved state of the superficial layerof the part, which facilitates the subsequent carburization treatment.

In an alternative form of pre-treatment, the part is given a metalliccoating having a thickness comprised between 1 and microns deposited onits surface, either by electrolysis or by metallization under vacuum, orby any other suitable means, this metallic layer being then dilfused ata temperature comprised between 600 and 850 C. for a time of between 1and hours. This treatment produces at the surface of the part a layerhaving a melting temperature higher than that of titanium or of thetitanium alloy of which it is composed, which makes it possible to carryout the carburization treatment at a higher temperature. The metaldeposited is chosen so that its melting temperature is higher than thatof titanium and that the diagram of solidification of the mixture of thetitanium with the metal does not have any eutectic. According to theinvention, the metal deposited is molybdenum, tungsten or platinum.

For the surface preparation, the part is subjected to one or more of thefollowing operations:

(1) The part is de-greased in a chlorated solvent or preferably inacetone;

(2) The part is subjected to chemical attack, for example by immersingit for 10 seconds in a bath having the following composition:

Percent by volume Hydrofluoric acid at 48% 18.5 Concentrated nitric acid2.5 Water 79.0

(3) The part is subjected to a super-finish of the vapour-blast type;

(4) The part is de-gasified under a vacuum of at least 10* millimetre ofmercury at a temperature comprised between 600 and 650 C. for a periodexceeding 5 hours.

The operation No. 1 is intended to remove the greater part of the stainsand dirt from the part. Operations No. 2 and No. 3 have the purpose ofremoving the coating of oxide which may soil the surface. Operation No.4 removes from the part the gases which it has absorbed and which mayadversely affect the carbonization treatment.

The present invention has also for its object the parts of titanium ortitanium alloy obtained by the above method and having a largesuperficial thickness of titanium carbide, greater than 5 microns, whichpermit of excellent performances.

Forms of embodiment of the invention are described below, by way ofnon-limitative examples, reference being made to the accompanyingdrawings, in which:

FIG. 1 illustrates the type of medium adopted for the carburization in afirst example;

FIG. la illustrates the method of heating adopted for the carburizationin the first example;

FIG. lb is a diagrammatic view of an apparatus for effecting thecarburization following the first example;

FIG. 2 illustrates the method of de-greasing a part with a view to itscarburization following a second example;

FIG. 2a illustrates a method of super-finishing the part by means of avapour blast according to the second example;

FIG. 2b illustrates a method of pre-treatment of the part following thesecond example;

FIG. 2c illustrates the type of medium adopted for the carburizationfollowing the second example;

FIG. 2d illustrates the method of heating adopted for the carburizationfollowing the second example;

FIG. 2e is a diagrammatic view of an apparatus for carrying into effectthe carburization following the second example;

FIG. 3 illustrates a method of de-greasing a part with a view to itscarburization following a third example;

FIG. 3a illustrates a method of degasifying this part following thethird example;

FIG. 3b illustrates the type of medium adopted for the carburizationaccording to the third example;

FIG. 3c illustrates the method of heating adopted for the carburizationfollowing the third example;

FIG. 3d is a diagrammatic view of an apparatus for carrying into effectthe carburization following the third example;

FIG. 4 illustrates a pre-treatment of a part for the purpose of itscarburization following a fourth example.

FIG. 4a illustrates a method of diffusion under vacuum applied to thispart in accordance with the fourth example;

FIG. 4b illustrates the type of medium adopted for the carburizationaccording to the fourth example;

FIG. 40 illustrates the method of heating utilized for the carburizationfollowing the fourth example;

FIG. 4d is a diagrammatic view of an apparatus for carrying out thecarburization according to the fourth example.

EXAMPLE 1 Reference will be made to FIGS. 1, la and lb.

There are mixed together in parts by weight, of carbon disulphide and 5%of carbon tetrachloride.

In the liquid thus obtained and stirred, there is put in suspensioncolloidal graphite at the rate of 50 grams per litre, which gives themixture 11 (see FIG. 1).

In a receptacle 12 (FIG. 1b) filled with the mixture 11, there is placeda Faville test-sample 13 made from a titanium alloy TA 6 V and having adiameter of 6.5 mm. and a length of 40 mm.

Around this test-sample is arranged a copper inductor 14 composed ofeight turns of 6- mm. in diameter, wound on a cylinder on 40 mm. indiameter.

The inductor is supplied with a power of 16 kw. at a frequency of 1megacycle. The heating period does not exceed 2 seconds. This heating iscontinuous, as shown in the diagram of FIG. 1a, in which the time hasbeen plotted in abscissae 0t and the power applied to the inductor inordinates OP.

It is found that after the treatment, the test-sample of TA 6 V alloyhas been carburized to a depth of microns. Its maximum hardness thusobtained, measured with a micro-hardness-meter under a load of 50 grams,is 1900 HV.

EXAMPLE II Reference will be made to FIGS. 2, 2a, 2b, 2c, 2d and 2e.

A test-sample 20 similar to the test-sample 13 of Example 1, is firstsubjected to de-greasing with acetone 21 (see FIG. 2) and a finishingtreatment by vapour-blast 22 (FIG. 2a), and is pre-treated by beingplaced in a chamber filled with graphite 23 (FIG. 2b) heated for 10hours at a temperature of 750 C.

The test-sample 20 is then placed in a chamber 24 (FIGS. 2c and 2e) atthe centre of a double-winding inductor 25 consisting of two concentriclayers of eight turns each of copper tube of 4 mm. in diameter, wound oncylinders of 25 mm. and 45 mm. in diameter.

A flow-rate of 20 litres per minute of a neutral gas, preferably argon,at a temperature of 120 C. is introduced at an inlet 27 of a mixer 26which receives through another inlet 28 a flow of 4 litres per minute ofcarbon disulphide vapour superheated to 120 C.

The gaseous mixture thus obtained is extracted from the mixer 26 and isintroduced at 29 into the chamber 24.

The treatment according to the invention is then effected with thefollowing characteristics:

Power available at the inductor: 20 kw.', Frequency: 0.5 megacycle;Duration of heating: 0.8 second.

The heating (see FIG. 2d) is continuous as in Example 1.

After treatment, the test-sample shows in micrographic section a uniformand adherent layer of 0.02 mm. in thickness of titanium carbide, thehardness of which, measured on the micro-hardness-meter under a load of50 grams, is 2400 HV.

EXAMPLE III Reference will be made to FIGS. 3, 3a, 3b, 3c and 3d.

The carrying out of the invention in accordance with the present ExampleIII has the advantage of providing a very accurate control of thethickness of the layer of carbide.

The test-sample 30, similar to the test-sample of the previous examples,is de-greased with acetone at 31 (see FIG. 3) and then degasified byheating to 800 C. in a vacuum furnace 32 (FIG. 3a). It is then placed ina chamber 33 (FIGS. 3b and 3d) at the centre of an inductor 34 similarto the inductor 25 of Example II.

A flask 35 containing liquid carbon disulphide is coupled to the lowerextremity of the chamber 33- by an electro-magnetic valve 36. From theupper extremity of the chamber 33 passes a tube which conducts thecarbon disulphide vapours into a condenser 37. The assembly comprisingthe flask, the valve and the chamber is enclosed in a casing 38 which ismaintained at a temperature of 55 C.

The treatment in accordance with the invention is then carried out asfollows: the valve 36 opens for 0.1 second in order to permit thepassage of about 15 cubic centimetres of carbon disulphide vapour. Whenthe valve is closed, a power of 30 kw. supplied by a current of 1.5magacycles is applied for 0.1 second to the inductor 34. The cycle isrepeated every 15 seconds for minutes. The excess of carbon disulphidevapour which has not taken part in the reaction is condensed in thecondenser 37 and is collected in a flask 39.

There can be seen in FIG. 3c a diagram showing this form ofdiscontinuous heating.

The test-samples thus treated show by micrographic examination, aperfectly uniform layer of 56 microns having great uniformity and ahardness of 2000 HV under 50 grams.

EXAMPLE IV Reference will now be made to FIGS. 4, 4a, 4b, 4c and 4d.

A test-sample 40 taken from an alloy TA 6 V of 12 mm. in diameter and 80mm. in height is coated by electrolytic treatment at 41 (FIG. 4) with anelectrolytic deposit of molybdenum of 7 microns in thickness and is thensubjected at 42 (FIG. 4a) to a diffusion treatment at a temperature of760 C. for a period of 18 hours under a vacuum of mm. of mercury.

The test-sample 40 is then placed (FIGS. 4b and 4d) in a chamber 41 atthe centre of an inductor 44 constituted by two layers of 10 turns ofcopper tube of 4 mm.

in diameter. The height of the inductor is 60 mm. and the respectivediameter of the layers are 30 mm. and 50 mm.

At the upper extremity of the chamber, a spraying device 45 atomizesliquid carbon tetrachloride.

The treatment according to the invention is then carried out with thefollowing characteristics:

Power supplied to the inductor: 70 kw.; Frequency of the current: 1.5megacycles; Period of heating: 0.7 second.

The heating is continuous, as shown in FIG. 4c.

The carburized layer thus obtained has a thickness of 60 microns and ahardness of 1700 HV at 50 grams.

What I claim is:

1. A method for producing a layer of titanium carbide at the surface ofa part of titanium or titanium alloy, in which said part of titanium ortitanium alloy is subjected, in a fluid medium with a base of a carboncompound selected from the group consisting of carbon disulphide, carbontetrachloride, and mixtures thereof, to a thermal treatment of shortduration by high frequency current, bringing the surface of said part toa temperature higher than 1100 C.

2. A method as claimed in claim 1, in which the surface of said part isbrought to a temperature higher than 1350 C.

3. A method as claimed in claim 1, in which said medium furthercomprises graphite, sulphur or alkaline carbides.

4. A method as claimed in claim 1, in which said fluid medium is aliquid filling a chamber adapted to receive said part to be treated.

5. A method as claimed in claim 3, in which said liquid is agitated andreceives, in suspension, carbon, sulphur or alkaline carbides.

6. A method as claimed in claim 1, in which said fluid medium is aliquid atomized in the form of a spray on the part to be treated.

7. A method as claimed in claim 1, in which said fluid medium isconstituted by a flow in the vapour state at a temperature higher than50 C.

8. A method as claimed in claim 7, in which said flow is conveyed by aneutral gas.

9. A method as claimed in claim 7, in which said flow passes through abed of graphite, sulphur or alkaline carbides.

10. A method as claimed in claim 7, in which said flow is effected inshort and repeated puffs.

11. A method as claimed in claim 1, in which the heating of said part isproduced by electric currents induced in the body of said part by aninductor through which passes an electric current at a frequency of atleast 0.2 megacycle and preferably of at least 0.4 megacycle, supplyinga power of at least 40 kw./ sq. dm. for a brief pre-determined durationless than 3 to 5 minutes and preferably less than seconds.

12. A method as claimed in claim 11, in which the electric power isapplied to said inductor continuously during said pre-deter-minedduration.

13. A method as claimed in claim 11, in which the electric power isapplied to said inductor in a discontinuous manner during the course ofsuccessive spacedapart periods of time, the total of which correspondsto said pre-determined duration.

14. A method as claimed in claim 1, in which said part is subjected to apre-treatment by being heated to a temperature comprised between 600 and850 C. for a period of between 1 and 20 hours in a chamber filled withgraphite.

15. A method as claimed in claim 1, in which said part is subjected to apre-treatment by being coated with a layer of metal such as molybdenum,tungsten or platinum, said layer having a thickness comprised between 1and 10 microns, and being then diffused at a temperature of between 600and 850 C. for a time comprised between 1 and 20 hours.

16. A method as claimed in claim 1, in which said part is subjectedbefore treatment to a surface prepara tion by de-greasing in a solventsuch as acetone.

17. A method as claimed in claim 1, in which said part is subjectedbefore treatment to a surface preparation by immersion in a bathcomprising hydrofluoric acid and nitric acid.

18. A method as claimed in claim 1, in which said part is subjectedbefore treatment to a surface preparation by a vapour-blast.

19. A method as claimed in claim 1, in which said part is subjectedbefore treatment to a surface preparation by degasifying under a vacuumof at least 10- millimetre of mercury at a temperature comprised between600 and 850 C. for a time longer than 5 hours.

References Cited UNITED STATES PATENTS 2,865,797 12/1958 McCawley14820.3 2,892,743 6/1959 Griest et al. 148-203 3,314,827 4/1967 De Vries148-20.3X

OTHER REFERENCES Hanzel, R. W., Surface Hardening Processes for Titaniumand its Alloys, Metal Progress, March 1964, pp. 89-96.

L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant ExaminerUS. Cl. X.R. 14820.6, 32

